In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed. The European Conference of Postal and Telecommunications Administrations (CEPT) originally standardized the E-carrier system, which revised and improved the earlier American T-carrier technology, and this has now been adopted by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T). This is now widely used in almost all countries outside the USA, Canada and Japan.
The E-carrier standards  form part of the Plesiochronous Digital Hierarchy (PDH) where groups of E1 circuits may be bundled onto higher capacity E3 links between telephone exchanges or countries. This allows a network operator to provide a private end-to-end E1 circuit between customers in different countries that share single high capacity links in between.
In practice, only E1 and E3 versions are used. Physically E1 is transmitted as 32 timeslots and E3 512 timeslots, but one is used for framing and typically one allocated for signalling call setup and tear down. Unlike Internet data services, E-carrier systems permanently allocate capacity for a voice call for its entire duration. This ensures high call quality because the transmission arrives with the same short delay (latency) and capacity at all times.
E1 circuits are very common in most telephone exchanges and are used to connect to medium and large companies, to remote exchanges and in many cases between exchanges. E3 lines are used between exchanges, operators and/or countries, and have a transmission speed of 34.368 Mbit/s.
An E1 link operates over two separate sets of wires, usually twisted pair cable. A nominal 3 volt peak signal is encoded with pulses using a method avoiding long periods without polarity changes. The line data rate is 2.048 Mbit/s (full duplex, i.e. 2.048 Mbit/s downstream and 2.048 Mbit/s upstream) which is split into 32 timeslots, each being allocated 8 bits in turn. Thus each timeslot sends and receives an 8-bit PCM sample, usually encoded according to A-law algorithm, 8000 times per second (8 x 8000 x 32 = 2,048,000). This is ideal for voice telephone calls where the voice is sampled into an 8 bit number at that data rate and reconstructed at the other end. The timeslots are numbered from 0 to 31.
One timeslot (TS0) is reserved for framing purposes, and alternately transmits a fixed pattern. This allows the receiver to lock onto the start of each frame and match up each channel in turn. The standards allow for a full Cyclic Redundancy Check to be performed across all bits transmitted in each frame, to detect if the circuit is losing bits (information), but this is not always used.
One timeslot (TS16) is often reserved for signalling purposes, to control call setup and teardown according to one of several standard telecommunications protocols. This includes Channel Associated Signaling (CAS) where a set of bits is used to replicate opening and closing the circuit (as if picking up the telephone receiver and pulsing digits on a rotary phone), or using tone signalling which is passed through on the voice circuits themselves. More recent systems used Common Channel Signaling (CCS) such as ISDN or Signalling System 7 (SS7) which send short encoded messages with more information about the call including caller ID, type of transmission required etc. ISDN is often used between the local telephone exchange and business premises, whilst SS7 is almost exclusively used between exchanges and operators. In theory, a single SS7 signaling timeslot can control up to 4096 circuits per signalling channel using a 12-bit Channel Identification Code (CIC), thus allowing slightly more efficient use of the overall transmission bandwidth because additional E1 links would use all 31 voice channels. ANSI uses a larger 14-bit CIC and so can accommodate up to 16,384 circuits. In most environments, multiple signalling channels would be used to provide redundancy in case of faults or outages.
Unlike the earlier T-carrier systems developed in North America, all 8 bits of each sample are available for each call. This allows the E1 systems to be used equally well for circuit switch data calls, without risking the loss of any information.
Link An unidirectional channel residing in one timeslot of a E1 or T1 Line, carrying 64 kbit/s (64'000 bit/s) raw digital data.
Line An unidirectional E1 or T1 physical connection.
Trunk A bidirectional E1 or T1 physical connection.
The PDH based on the E0 signal rate is designed so that each higher level can multiplex a set of lower level signals. Framed E1 is designed to carry 30 E0 data channels + 1 signalling channel, all other levels are designed to carry 4 signals from the level below. Because of the necessity for overhead bits, and justification bits to account for rate differences between sections of the network, each subsequent level has a capacity greater than would be expected from simply multiplying the lower level signal rate (so for example E2 is 8.448 Mbit/s and not 8.192 Mbit/s as one might expect when multiplying the E1 rate by 4).
Note, because bit interleaving is used, it is very difficult to demultiplex low level tributaries directly, requiring equipment to individually demultiplex every single level down to the one that is required.
T-carrier and E-Carrier systems North American Japanese European (CEPT) Level zero (channel data rate) 64 kbit/s (DS0) 64 kbit/s 64 kbit/s First level 1.544 Mbit/s (DS1) (24 user channels) (T1) 1.544 Mbit/s (24 user channels) 2.048 Mbit/s (32 user channels) (E1) (Intermediate level, T-carrier hierarchy only) 3.152 Mbit/s (DS1C) (48 Ch.) – – Second level 6.312 Mbit/s (DS2) (96 Ch.) (T2) 6.312 Mbit/s (96 Ch.), or 7.786 Mbit/s (120 Ch.) 8.448 Mbit/s (128 Ch.) (E2) Third level 44.736 Mbit/s (DS3) (672 Ch.) (T3) 32.064 Mbit/s (480 Ch.) 34.368 Mbit/s (512 Ch.) (E3) Fourth level 274.176 Mbit/s (DS4) (4032 Ch.) 97.728 Mbit/s (1440 Ch.) 139.264 Mbit/s (2048 Ch.) (E4) Fifth level 400.352 Mbit/s (DS5) (5760 Ch.) 565.148 Mbit/s (8192 Ch.) 565.148 Mbit/s (8192 Ch.) (E5)
Note 1: The DS designations are used in connection with the North American hierarchy only. Strictly speaking, a DS1 is the data carried on a T1 circuit, and likewise for a DS3 and a T3, but in practice the terms are used interchangeably.
Note 2: There are other data rates in use, e.g., military systems that operate at six and eight times the DS1 rate. At least one manufacturer has a commercial system that operates at 90 Mbit/s, twice the DS3 rate. New systems, which take advantage of the high data rates offered by optical communications links, are also deployed or are under development. Higher data rates are now often achieved by using synchronous optical networking (SONET) or synchronous digital hierarchy (SDH).
Note 3: A DS3 is delivered native on a copper trunk. DS3 may be converted to an optical fiber run when needing longer distances between termination points. When a DS3 is delivered over fiber it is still an analog type trunk connection at the termination points. When delivering data over an OC3 or greater SONET is used. A DS3 transported over SONET is encapsulated in a STS-1 SONET channel. An OC-3 SONET link contains 3 STS-1s, and therefore may carry 3 DS3s. Likewise, OC-12, OC-48, and OC-192 may carry 12, 48, and 192 DS3s respectively.
- D 0 (DS0)
- Digital Signal 1 (DS1, T1)
- HDB3 Encoding scheme
- List of device bandwidths
- Plesiochronous Digital Hierarchy
- Time-division multiplexing
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